Even though Windows 8 has only been out for around a month, already more than 20,000 apps have made their way into the operating system’s app store. They are barreling their way into the Windows 8 app storefront at a rate of about 500 per day.

But the new platform isn’t exactly a cash cow for Microsoft just yet. In fact, of those 20,000 or so apps, about 18,000 of them are free. Apparently, the current free-to-paid ratio on the Windows 8 app platform is about 87-to-13.

This is the same problem Google’s app marketplace has been facing since day one. Even though there are 600,000 Android apps, not too far off Apple’s 700,000 active App Store apps (over one million lifetime), developers still flock heavily to the latter. Why is that? Because the App Store is where you can make money. Android is where you go if you’re an independent developer and just want to see your app appear in Google Play.

Every new marketplace has to start somewhere, though, and Microsoft is building the Windows 8 app interface from the ground up. If it keeps growing at the same rate, there could be around 40,000 apps by the end of the year, which is pretty impressive regardless of whether they’re free or not.

And if you do a more favorable apples-to-apples comparison (no pun intended), Apple’s Mac App Store only added 10,000 apps for the 15-month period from January 2011 to April 2012. So for Windows 8, 20,000 apps in a month is pretty good.

As a side note, those numbers represent global totals. Unlike other app platforms, Microsoft restricts which apps appear in which regions. In the US, for example, less than 13,000 apps are available. Canada surprisingly has the largest number of apps, at around 14,000.